1. Field of the Invention
The invention relates to voltage-controlled oscillators. More particularly, the invention relates to a voltage-controlled oscillator, suitable for use in phase-lock loop circuits and other circuits, whose gain is proportional to its operating frequency.
2. Description of the Related Art
A voltage-controlled oscillator (VCO) is a device that generates a periodic output signal whose frequency is a function of the VCO input voltage. The VCO has a characteristic gain, which often is expressed as a ratio of the VCO output frequency to the VCO input voltage. Stated another way, the VCO gain is the slope of an operating curve or set of operating curves of the VCO, where the x-axis is VCO input voltage and the Y-axis is VCO output frequency.
Voltage-controlled oscillators are used in many circuits, including phase-lock loop (PLL) circuits. A PLL circuit is a circuit that generates a periodic output signal that has a constant phase relationship with a periodic input signal. PLL circuits are used in many types of measurement, electromechanical control, microprocessor and communication applications.
Typically, a conventional PLL circuit includes a phase/frequency detector (PFD), a filter and a voltage-controlled oscillator (VCO). The PFD measures differences in phase between an incoming signal and a feedback signal from the VCO. Any detected differences generate an error signal, which is filtered and then supplied to the VCO. The VCO uses the error signal to reduce the frequency difference between the feedback signal and the incoming signal.
U.S. Pat. No. 6,114,920 discloses a PLL circuit with an oscillator architecture that includes autotrim. Autotrim is a feature or procedure that calibrates the center frequency of the VCO during a power-up or reset of the PLL circuit. This autotrim feature allows the oscillator to operate across a relatively wide output frequency range while still maintaining a relatively low VCO gain.
However, in the VCO architecture in the PLL circuit disclosed in U.S. Pat. No. 6,114,920, the VCO gain is not proportional to the operating frequency of the VCO. Thus, such VCO architecture may not be suitable for applications in which the VCO gain needs to be proportional to the operating frequency, e.g., 10,000 ppm/V (ppm/V=percent parts per million per unit voltage), while the VCO still needs to be able to operate across a relatively wide range of operating frequencies.
Accordingly, it would be desirable to have available a VCO in which the gain of the VCO is proportional to the operating frequency of the VCO.